This invention relates to devices for measuring electrostatic field strength and particularly to devices for detecting and indicating the strength of an electrostatic field in the vicinity of a hovering aircraft such as a helicopter.
It is well known that the operation of an aircraft often tends to cause an accumulation of electrostatic charges through various effects including atmospheric conditions and the motion of the aircraft or parts thereof through the atmosphere. The accumulation of the electrostatic charges tends to build up potentials than can become dangerous and can cause radio frequency interference, electrical shock to persons connected with the aircraft, and unwanted ignition of fuel and armament. It therefore becomes necessary to provide an apparatus for measuring the strength of the electrostatic field between the aircraft and the surrounding environment in order to determine whether or not the electrostatic potential of the aircraft is at a dangerous level.
While several methods of measuring electric fields have been developed which have enjoyed varying degrees of success, it has been difficult to reliably determine the potential differences between a charged body and surrounding structures when the intervening atmosphere includes any substantial number of charged particles or ions. For example, when triboelectrically charged particles from a rotating helicopter rotor mixed with the ions resulting from corona discharge from the helicopter rotor are forced downward by the flow of air sustaining the helicopter in flight, the helicopter is immersed in a sea of charged particles and ions which all but prevent any meaningful measurement of electrical fields in the vicinity of the helicopter. Likewise, ions carried by a flowing fluid such as fuel through high velocity fuel pumps can create similar problems if potential difference measurements are attempted in an effort to reduce fire and explosion hazards. Often in such situations, ions and charged particles can become bound to nonconductive portions of the structure in the vicinity of the electric field sensor, resulting in apparent field modifications having little or no relation to the actual potential differences between the body as a whole and surrounding stuctures. Examples of electrostatic field measuring devices particularly adapted for use in environments of this type are illustrated in U.S. Pat. Nos. 3,260,893; 3,600,632; 3,812,419; 3,857,066; and 3,874,616. Attempts have been made to use the sensed field to signal an appropriate device to discharge the charged or charging body. The electrical discharge of aircraft is illustrative of such attempts and has been reasonably successful when the aircraft has enjoyed a sufficent velocity vector with respect to any triboelectrically charged particles and ion products, but has generally been unsuccessful in the case of hovering aircraft since the fluid borne electric charges adversely affect the electric field sensor's ability to accurately detect either the polarity or magnitude of the charge on the aircraft.
The electrostatic fields have typically been measured with the use of conventional electrometers of the so called field mill type. Generally, a field mill is a device for measuring electrostatic field strength on the basis of charges induced on a sensor electrode within the field mill which is either electrically or mechanically alternately covered and exposed to the electrostatic field sought to be measured. The induced charges on the sensor electrode of the field mill are proportional to the external field impinging on the sensor electrode. With appropriate phase convergence, both the strength and polarity of the electrostatic field sought to be measured can be determined.
It is an object of this invention to combine a conventional field mill with other appropriate apparatus so as to more accurately measure the electrostatic field surrounding a body. It is a further object of this invention to combine a conventional field mill with a sensing means postitioned so as to intercept or "short out" at least a portion of the equal potential lines surrounding the body and thus more accurately determine the electrical field strength. Yet a further object of this invention is to provide means by which the sensor means may be maintained free from any ion or charged particle buildup due to the presence of ions and charged particles in the surrounding fluid. A further object of this invention is to utilize an electric field sensor as herein disclosed to indicate the electric charge state of a body in such a manner that the indication may be used to operate an active discharge system to discharge the charged bodies.